1. Field of the invention
The present invention relates to a 2-part hair dyeing agent, and more specifically to a 2-part durable hair dyeing agent emanating no irritating odor, causing little damage to hair and requiring comparatively short time for the dyeing.
2. Description of the prior art
Hair dyeing agents are classified into two large groups: durable hair dyeing agents which do not decolor by shampooing, and temporary hair dyeing agents which decolor by shampooing, the former being more widely used at present.
Conventional durable hair dyeing agents contain as essential components a first agent comprising an alkali and a hair dye such as an oxidation dye or direct dye and a second agent comprising hydrogen peroxide.
The first and second agents are mixed with each other just before use and then applied to hair. The dye molecules penetrate into hair, and nascent oxygen generating from hydrogen peroxide decolors (bleaches) hair. Where an oxidation dye is used, the nascent oxygen also oxidizes and polymerizes the oxidation dye to develop color. The alkali contained in the first agent swells hair, thereby facilitating the penetration of dye molecules into hair, and at the same time promotes generation of nascent oxygen from hydrogen peroxide, thereby increasing the effect of dyeing. Ammonia, mono-, di- and trialkanolamine and the like are generally used as the alkali.
However, while these alkalis used increase the dyeing effect, the following problems associated with their use have been pointed out.
(1) The pH of the first agent is generally so adjusted that the 2-part hair dyeing agent obtained by mixing the first and second agents will be in the range of from 9 to 11. In this pH range, however, free alkali is present in a large amount, which irritates skin and causes hair to be damaged.
(2) Where ammonia, which is volatile, is used as alkali, its irritating odor is disagreeable.
Where mono-, di- or triethanol amine is used, which is only a little volatile and hence not so disagreeable, the alkali remains inside hair after the use and attacks keratin in hair, thereby causing the hair to be damaged.
Japanese Patent Application Laid-open No. 85745/1973 discloses a 2-part hair dyeing agent utilizing as alkali an ammonium salt and a base instead of aqueous ammonia, which is said to be successful in suppressing the above hair damage and irritating or disagreeable odor.
The 2-part hair drying agent according to the invention described in the above application comprises, for example, a first agent consisting of a hair dye and a base such as sodium hydroxide and a second agent consisting of urea hydrogen peroxide as an oxidizing agent and an ammonium salt such as ammonium chloride.
This hair dyeing agent, when used after mixing is said to emanate no irritating odor since the ammonium salt and the base gradually generate ammonia in a minimum amount required, thus being different from the usual cases where aqueous ammonia is directly incorporated in a first agent.
However, this 2-part hair dyeing agent comprising an ammonium salt and a base as alkali must generate ammonia in bleaching (decoloring) and dyeing, and hence does not differ from conventional 2-part dyeing agents, in that ammonia is still being used for bleaching and dyeing. The use of this agent therefore cannot fully eliminate disagreeable odor, but simply reduce it to a certain degree.
Furthermore, this agent creates another problem, namely, the problem of requiring a considerably long time of about 40 minutes, since ammonia generates only gradually, as shown in the above application.
As a result of an intensive study, the present inventors have found that the above problems can all be solved by providing a first agent comprising a specific compound capable of promoting generation of nascent oxygen and a hair dye and a second agent comprising hydrogen peroxide, and further incorporating an alkali that generates substantially no irritating odor into the first agent and incorporating a buffer solution to stabilize the hydrogen peroxide in the second agent, both agents being prepared in such amounts as to permit the pH to be in a nearly neutral zone, after the mixing of the two agents. The present invention was completed based on this finding.